System including a host connected serially in a chain to one or more memory modules that include a cache

ABSTRACT

A system including a host coupled to a serially connected chain of memory modules. In one embodiment, at least one of the memory modules includes a cache for storing data stored in a system memory.

This application claims the benefit of U.S. Provisional Application No. 60/470,078 filed May 13, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to computer system memory and, more particularly, to memory module configurations and the memory subsystem topology.

2. Description of the Related Art

Many computer systems employ a main system memory that may be configured dependent upon the needs of an end user. In such systems, a motherboard or system board may include a number of memory expansion sockets. One or more small circuit boards, referred to as memory modules, may be inserted into the sockets as needed to increase the memory capacity of the computer system. Each of the memory modules typically includes multiple memory devices that provide a given amount of memory capacity. The memory devices are usually implemented using some type of dynamic random access memory (DRAM). Some examples of DRAM types include synchronous DRAM (SDRAM) as well as the various types of double data rate SDRAM (DDR SDRAM).

In conventional computer systems, the memory modules are connected to a memory/DRAM controller via a memory bus that includes address, control (including clock signals), and data signals. In some computer systems, the address, control and data signals may be multiplexed and thus share the same sets of wires. In other computer systems, the address, control and data signals may use separate wires. In either case, each of the address and control signals are routed to each expansion socket such that the memory modules, when inserted, are connected in parallel to the memory/DRAM controller. There are several drawbacks to parallel bus arrangements. For example, depending on the topology of the interconnect, the maximum memory bus speed may be limited due to transmission line effects such as signal reflections. In some systems, the memory/DRAM controller may reside on the same integrated circuit (IC) chip as the system processor, while in other systems the memory/DRAM controller may reside in one IC (e.g., a Northbridge) of a chipset.

In addition, many conventional computers systems employ processors that use an external cache memory. In many such systems, the external cache memory interface uses a large number of address and data pins. Furthermore, increasing the size of the external cache may require a system redesign and so the external cache may not be configurable by an end user.

Although the operating speed of computer system processors continues to increase, the relative performance of the main system memory has not increased at the same rate. This may be due, at least in part, to the incremental improvement in the bandwidth of the memory bus architectures described above.

SUMMARY

Various embodiments of a system including one or more memory modules are disclosed. In one embodiment, a host is coupled to a serially connected chain of memory modules. At least one memory module includes a cache for storing data stored in a system memory.

In one specific implementation, the system memory may include a respective plurality of memory chips mounted on each memory module. In addition, each memory module may include a memory control hub including a controller configured to determine whether data associated with a received memory request is stored within the cache.

In another specific implementation, a given cache may include a storage for storing cache tags corresponding to the data stored within the given cache. In such an implementation, the controller is configured to access the cache in response to determining that the data associated with the received memory request is stored within the cache.

In another specific implementation, the host includes a memory controller configured to generate memory requests to the memory modules. The memory controller may include a storage for storing cache tags corresponding to data stored within each cache.

In yet another implementation, the memory control hub includes a controller that may be configured to access the memory chips in response to receiving a memory command having a memory address that matches a memory address associated with the memory chips. In addition, the controller may be configured to access the cache in response to receiving a memory command having a memory address that matches a memory address associated with the cache.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of a system including a serially connected chain of system memory modules.

FIG. 2 is a block diagram of one embodiment of a memory module of FIG. 1 including one cache implementation.

FIG. 3 is a block diagram of another embodiment of a memory module of FIG. 1 including another cache implementation.

FIG. 4 is a block diagram of one embodiment of a system including a serially connected chain of cache memory modules.

FIG. 5 is a diagram of one embodiment of a memory read packet.

FIG. 6 is a diagram of one embodiment of a cache memory read packet.

FIG. 7 is a block diagram of one embodiment of a computer system.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must). The term “include” and derivations thereof mean “including, but not limited to.” The term “connected” means “directly or indirectly connected,” and the term “coupled” means “directly or indirectly coupled.”

DETAILED DESCRIPTION

Turning now to FIG. 1, a block diagram of one embodiment of a system including a serially connected chain of memory modules is shown. System 50 includes a host 100 coupled to a system memory 125 via a memory link 1I A. System 50 may be configured to operate as part of a computing device such as a computer system or server system, for example. System memory 125 includes a memory module 150A coupled to a memory module 150B via a memory link 110B. Memory module 150B is shown coupled to a memory link 110C, which may be coupled to an additional memory module (not shown) as desired to form a serially connected chain of memory modules that is coupled to host 100. It is noted that although two memory modules are shown in the chain, it is contemplated that one or more memory modules may be connected in this manner. It is further noted that components including a reference number followed by a reference letter may be referred to generally by the reference number alone. For example, when referring generally to all memory modules, reference may be made to memory module 150.

In the illustrated embodiment, memory module 150A includes a memory control hub 160A, which is coupled to a plurality of memory devices that are designated memory chip 171A through 171N, where N may be any number, as desired. In one embodiment, memory control hub 160A may be coupled to the memory chips via any type of memory interconnect. For example, in one embodiment, the memory interconnect may be a typical address, control and data bus configuration.

Similarly, memory module 150B includes a memory control hub 160B, which is coupled to a plurality of memory devices that are designated memory chip 181A through 181N, where N may be any number, as desired. In one embodiment, memory control hub 160B may be coupled to the memory chips via any type of memory interconnect as described above. It is noted that each of memory chips 171A through 171N and 181A through 181N may be any type of memory device such as a memory device in the DRAM family of memory devices, for example.

In the illustrated embodiment, memory links 110A-110C form a memory interconnect. In one embodiment, each of memory links 110A-110C forms a point-to-point memory interconnect that is implemented as two sets of unidirectional lines. One set of unidirectional lines is referred to as a downlink and is configured to convey transactions away from host 100 in a downstream direction. The other set of unidirectional lines is referred to as an uplink and is configured to convey transactions toward host 100 in an upstream direction. In addition, in one embodiment, each set of unidirectional lines may be implemented using a plurality of differential signal pairs. In one embodiment, each memory link 110 includes an 18-bit downlink and a 16-bit uplink, where each bit is a differential signal pair. As will be described in greater detail below in conjunction with the descriptions of FIG. 4 and FIG. 5, the memory interconnect formed by memory links 110 may be configured to convey packets.

In an alternative embodiment, each of memory links 110 may form a point-to-point memory interconnect that is implemented as one set of bi-directional lines. As such, transactions may flow both upstream and downstream on the set of bi-directional wires. In such an embodiment, the bi-directional lines may be implemented using a plurality of differential signal pairs. It is noted that in other embodiments, other signaling schemes may be used, such as multi-level signaling, for example.

Generally speaking, all transactions from host 100 flow downstream through all memory modules 150 on the downlink and all response transactions flow upstream from the responding memory module 150 through each upstream memory module 150 on the uplink. More particularly, in one embodiment, host 100 may request to retrieve or store data within system memory 125. In response to host 100 making a request, memory controller 105 initiates a corresponding transaction such as a memory read transaction or a memory write transaction, for example. Memory controller 105 transmits the transaction to system memory 125 via memory link 110A. In the illustrated embodiment, the transaction is received by memory control hub 160A of memory module 150A.

In response to receiving the transaction, memory control hub 160A is configured to transmit the received transaction to memory module 150B via memory link 110B without decoding or modifying the transaction. This is referred to as forwarding the transaction downstream. Thus, each transaction received on a downlink by a given memory control hub 160 of a given memory module 150 is forwarded to the next memory module 150 in the chain that is coupled to the downlink without decoding the transaction. In one embodiment, decoding of the transaction may occur in parallel with the forwarding of the transaction. In other embodiments, the decoding of the transaction may occur after the transaction has been forwarded.

Likewise, if memory controller 105 initiates a read request transaction, for example, the memory module 150 having the memory location corresponding to the address in the request will respond with the requested data. The response will be transmitted on the memory module's uplink toward host 100. If there are any intervening memory modules between the sending memory module and host 100, the intervening memory module will forward the response transaction on its uplink to either host 100 or the next memory module in the chain in an upstream direction. In addition, when the responding memory module is ready to send the response, it may inject the response into a sequence of transactions that are being forwarded upstream on the uplink.

In one embodiment, memory controller 105 may be configured to make requests to system memory 125 without knowledge of which of memory modules 150A and 150B a particular address is associated. For example, each of memory modules 150 may be assigned a range of memory addresses during a system configuration sequence. Each memory control hub 160 may include logic (not shown in FIG. 1) that may decode the address of an incoming request. Thus, a memory control hub 160 of a given memory module 150 may initiate a memory read cycle or memory write cycle to the memory chips on the given memory module 150 in response to decoding a memory request having an address that is in the address range assigned to the given memory module 150. As will be described in greater detail below in conjunction with the descriptions of FIG. 2 and FIG. 3, each memory control hub 160 may include a DRAM controller (not shown in FIG. 1) for initiating memory cycles to the memory chips to which it is connected.

In addition, in one embodiment, memory controller 105 may initiate a subsequent memory access request prior to receiving a response to a previous memory access request. In such an embodiment, memory controller 105 may keep track of outstanding requests and may thus process the responses in a different order than they were sent.

Further, in the illustrated embodiment, memory control hubs 160A and 160B include a cache memory designated 175A and 175B, respectively. Cache memories 175A-B may each serve as a cache memory for data stored elsewhere in the computing system. For example, as will be described in greater detail below, cache memories 175A-B may each serve as a cache memory for data stored within the respective memory chips of each memory module. Alternatively, cache memories 175A-B may each serve as a cache memory for data stored within other memory modules in the chain that may be further from the host and thus have longer latencies. Likewise, in computing systems having multiple processor nodes, cache memories 175A-B may serve as a cache memory for data stored in a remote processor node. In the embodiment described in conjunction with the description of FIG. 3, cache 175 includes storage for cache tags. However, in the embodiment described in conjunction with the description of FIG. 2, cache 175 does not include storage for cache tags. In such an embodiment, memory controller 105 may include cache tag storage (not shown). It is noted that although cache memories 175 are shown as part of memory control hub 160 (e.g., on the same device), it is contemplated that in other embodiments, cache memories 175 may be implemented on different devices than memory control hub 160.

The Memory Interconnect

The memory interconnect includes one or more high-speed point-to-point memory links such as memory links 110A-110C each including an uplink such as uplink 111A and a downlink such as downlink 112A, for example. As noted above, in one embodiment downlinks may be 18-bit links while uplinks may be 16-bit links. As such, an 18-bit downlink may include 16 control, address and data (CAD) signals, a busy signal and a Control (CTL) signal. A given uplink may include 16 control, address and data (CAD) signals. It is contemplated however, that in an alternative embodiment, an uplink such as uplink 211A may also include a CTL signal.

In addition to the high-speed links, other signals may be provided to each memory module 150. For example, in one embodiment, a reset signal, a power OK signal and a reference clock may be provided to each memory module 150 from host 100. Further, other signals may be provided between each memory module. For example, as described above, a next memory module present signal may be provided between memory modules.

Generally speaking, the types of transactions conveyed on memory links 110 may be categorized into configuration and control transactions and memory transactions. In one embodiment, configuration and control transactions may be used to configure memory control hub 160. For example, configuration and control transactions may be used to access configuration registers, assign a memory address range to a memory module or to assign a hub address to a memory control hub. Memory transactions may be used to access the memory locations within the memory chips (e.g., 171A-171N . . . 181A-181N). In addition, as described further below, certain memory transactions may be used to directly access cache 175.

Accordingly, in one embodiment, there are two types of addressing supported: hub addressing and memory addressing. Using hub addressing, eight hub bits identify the specific memory control hub being accessed. In one embodiment, a hub address of FFh may be indicative of a broadcast to all memory control hubs. Using memory addressing, each hub decodes the upper portion of the address bits to determine which hub should accept the request and the lower portion to determine the memory location to be accessed. In one embodiment, there are 40 address bits, although it is contemplated that other numbers of address bits may be used as desired.

As described further below, in certain embodiments, there may be an additional memory addressing type supported. The additional memory addressing type may be used to specifically access a cache memory located on a given memory module. For example, the last four entries of table 1, below, illustrate some exemplary cache access command codes.

In one embodiment, each of the memory links is configured to convey the transactions using one or more packets. The packets include control and configuration packets and memory access packets, each of which may include a data payload depending on the type of command the packet carries. As such, the sets of wires that make up memory links 110 may be used to convey control, address and data.

The packets may be generally characterized by the following: Each packet includes a number of bit positions which convey a single bit of information. Each packet is divided into several bit times and during a given bit time, all of the bit positions of the packet are sampled. As such, the control information and data share the same wires of a given link (e.g., CAD wires). As will be described in greater detail below, in one embodiment, packets are multiples of bit pairs and the first bit-time of every packet is sampled at an even bit-time. Packets begin with a control header that may be either one or two bit-pairs in length. In one embodiment, the first five bits of the control header is the command code. Table 1 below illustrates the various types of packets and their associated command codes. It is noted however, that the actual codes shown in column one are for illustrative purposes and that other codes may be used for each given command. It is further noted that other commands and encodings may be used. TABLE 1 Packet types and command codes Header Length (bit- Normal Address Code times) Command Description Direction Response Type 00h — NOP Null Operation/Idle State Both — — 04h 2 AddrSet Address Set Down AddrAck Hub 05h 2 AddrAck Address Acknowledge Up — — 06h 2 Ack Acknowledge Up — — 07h 2 Nak Not Acknowledge/Error Up — — 08h 2 SRdResp Short Read Response Up — — 09h 2 LRdResp Long Read Response Up — — 0Ah 2 ConfigRd Configuration Read Down RdResp Hub 0Ch 2 ConfigWr Configuration Write Down Ack Hub 0Eh 2 DIMMCt1 DIMM Control Down Ack Hub 10h 4 SMemRd Short Memory Read Down RdResp/Ack Memory 11h 4 LMemRd Long Memory Read Down RdResp Memory 12h 4 BlkMemWr Block Memory Write Down Ack Memory 13h 4 SbytMemWr Short Byte Memory Write Down Ack Memory 14h 4 LbytMemWr Long Byte Memory Write Down Ack Memory 15h 4 CacheDataRd Cache Read Down RdResp M/Cache 16h 4 CacheDataWr Cache Write Down Ack M/Cache 17h 4 CacheTagRd Cache Tag Read Down RdResp M/Cache 18h 4 CacheTagWr Cache Tag Write Down Ack M/Cache

Further, in one embodiment, packets (except NOP packets) are transmitted with an error detecting code (EDC). It is noted that in one embodiment, the EDC is a 32-bit cyclic redundancy code (CRC), although other embodiments may employ other EDC's as desired. Additionally, addresses are sent most significant bit-time first to speed decode within memory control hub 160 while data is sent least significant byte first. It is noted however, that other embodiments are contemplated in which the addresses may be sent least significant bit-time first and data my be sent most significant byte first. Packets may carry a payload of byte enables and/or data. Packets with no payload are referred to as header-only packets. In one embodiment, the size of the data short reads may be up to one half of a programmed cache line size. In addition, the size of the data for long reads and block writes may be up to the programmed cache line size. Further, the size of the data for byte writes may be a maximum of 64 bytes regardless of the cache line size setting.

In addition to the control header and command code information included within a packet, the CTL signal may be used to convey information about each packet. As illustrated in Table 2 below, some exemplary CTL encodings are shown. TABLE 2 CTL encodings for downstream use Even Odd Content of CAD 0 0 Data or Byte Enable Payload 1 1 Control Header 0 1 CRC for a Packet with Payload 1 0 CRC for a Header-Only Packet

Different values of CTL for the header and payload portions of a packet may provide enough information to allow header-only packets to be inserted within the payload of another packet. This may be useful for reducing the latency of read commands by allowing them to issue while a write packet is still being sent on the link. Table 3 illustrates an exemplary packet including a payload in tabular format. The packet in table 3 also shows that a header-only packet is inserted in the payload during bit times 4-7. It is noted however, that other packet encodings are possible and contemplated. For example, in other embodiments, a portion of the CRC bits may be transmitted during each bit time. TABLE 3 Packet with payload and header-only packet inserted within payload Bit-time CTL CAD 0 1 Headerl bits [15:0] 1 1 Headeri bits [31:16] 2 0 Databits [15:0] 3 0 Databits [31:16] 4 1 Header2 bits [15:0] 5 1 Header2 bits [31:16] 6 1 CRC2 bits [15:0] 7 0 CRC2 bits [31:16] 8 0 Data bits [47:32] 9 0 Data bits [64:48] 10 0 CRC1 bits [15:0] 11 1 CRC1 bits [31:16]

Referring to FIG. 2, a block diagram of one embodiment of a memory module such as the memory module illustrated in FIG. 1 is shown. Components that correspond to those shown in FIG. 1 are numbered identically for clarity and simplicity. Memory module 150 includes a memory control hub 160 coupled to memory chips 261A through 261N via z memory bus 265.

Memory control hub 160 includes a control unit 240 coupled to a DRAM controller 250. DRAM controller 250 is coupled to memory chips 261A-261N and to a cache memory 175. Control unit 240 includes an uplink control 241 and a downlink control 242. As noted above, memory bus 265 may be any type of memory interconnect. In the illustrated embodiment, memory control hub 160 is coupled to a memory link 110A in an upstream direction and a memory link 110B in a downstream direction. It is further noted that the frequency of operation of memory bus 265 may be independent of the frequency of operation of memory links 110. It is noted that although cache memory 175 is shown as part of memory control hub 160, in other embodiments, cache memory 175 may be separate from memory control hub 160, but may still be included on the same memory module.

In the illustrated embodiment, uplink control unit 241 may be configured to receive and forward packets received from another memory module downstream. The receiving and forwarding of the upstream packets creates an upstream transaction sequence. In addition, uplink control unit 241 may be configured to inject packets that originate within memory module 150 into the transaction stream.

In the illustrated embodiment, downlink control unit 242 may be configured to receive packets that originate at the host and if a memory module is connected downstream, to forward those packets to the downstream memory module. In addition, downlink control unit 242 may be configured to copy and decode the packets. In one embodiment, if the packets include an address that is within the range of addresses assigned to memory module 150 and the packet is a memory access request, downlink control unit 242 may pass the command associated with the packet to DRAM controller 250. However, if the packet is not a memory request, but is instead a configuration packet, downlink control unit 242 may pass the configuration command associated with the packet to the core logic of control unit 240 (not shown) for processing. It is noted that in one embodiment, if the packet does not include an address that is within the range of addresses assigned to memory module 150, memory control hub 160 may drop or discard the packet if memory module 150 is the last memory module in the chain.

In one embodiment, memory control hub 160 is configured to receive a module present signal (not shown), which when activated by a downstream memory module, indicates to an upstream memory module that there is a downstream memory module present. In such an embodiment, if memory control hub 160 receives a transaction and no downstream memory module is determined to be present, memory control hub 160 may drop the transaction. In addition, if no downstream memory module is determined to be present, a memory control hub 160 may power down the downstream transmit and receive circuits; thereby reducing power consumption and possibly radiated emissions.

As mentioned above, in one implementation, cache memory 175 of FIG. 2 is configured to cache frequently accessed data, whether that data is stored within memory chips 261A-N or some other place within the system. Thus, depending on whether the requested data is stored within cache memory 175, DRAM controller 250 is configured to initiate memory cycles to either to cache memory 175 or to memory chips 261A-261N in response to memory commands received by memory control hub 160. Alternatively, DRAM controller 250 may respond with a cache miss which may initiate a request to another memory in response to memory commands received by memory control hub 160. In one embodiment, cache memory 175 may be implemented using memory devices that are typically used for cache memory in a processor. For example, the memory devices may be in the static RAM (SRAM) or fast SRAM (FSRAM) family of devices.

In the illustrated embodiment, cache memory 175 is configured to store cache data while memory controller 105 includes storage for cache tags corresponding to the cache data stored within cache memory 175. In one implementation, memory controller 105 is configured to perform a tag lookup within a tag storage (not shown) prior to initiating a memory access request to system memory 125. In doing so, memory controller 105 determines whether the data is located in cache memory 175 or not. In either case, the access times associated with the request may be planned for and the responses scheduled accordingly.

As described above, each memory module may be associated with a particular address space. Thus, when a memory request is received that has an address associated with memory module 150, DRAM controller 250 is configured to generate read or write cycles to either cache 175 or memory chips 261A-N. In one embodiment, the address space associated with memory chips 261A-N may be different than the address space associated with cache memory 175. For example, the address space associated with memory chips 261A-N of all memory modules may be 00000000h through FFFFFFFFh, while the address space associated with of all cache memories may be 00000h through FFFFFh. To access the address space associated with memory chips 261A-N a memory access command code is used and to access the address space associated with cache memory 175, a cache access command code is used. Exemplary memory and cache read packets are described below in conjunction with the descriptions of FIG. 5 and FIG. 6, respectively. Since the memory controller 105 has determined whether the data resides in cache memory 175 or in DRAM chips 261A-N, memory controller 105 sends the access request using the correct addressing type.

In an alternative embodiment, the memory space associated with memory module 150 may include addresses associated with memory chips 261A-N as well as addresses associated with cache memory 175. For example, if the address space associated with memory module 150 includes addresses in the range 00000000h through 3FFFFFFF, the address space associated with cache memory 175 may be allocated to addresses 00000000h through 000FFFFFh and the remaining addresses may be allocated to memory chips 261A-N. Thus, if a memory request having an address in the DRAM range is received, DRAM controller 250 may access memory chips 261A-N. Conversely, if the received memory request has an address in the cache address range, DRAM controller 250 may access cache 175. In such an alternative embodiment, the type of packet may be a standard memory read or write packet including the requested address.

In one embodiment, DRAM controller 250 may include memory control logic (not shown) that may provide support for ensuring that cached data is written back to memory chips 261A-N. For example, DRAM controller 250 may provide a write back buffer and/or an eviction/victim buffer and support logic (not shown) for cache memory 175. In such an embodiment, DRAM controller 250 may implement an eviction algorithm such as a least recently used (LRU) algorithm, for example, for evicting data from cache memory 175. In one implementation, memory controller 105 may provide explicit write back instructions to DRAM controller 250.

As noted above in the description of FIG. 1, cache memory 175 of FIG. 2 may also be configured to cache frequently accessed data stored within a remote processor node (not shown in FIG. 2) or the memory chips of another downstream memory module. In such implementations, memory controller 105 may explicitly write data to cache memory 175 rather than writing to cache memory 175 as an artifact of writing to memory chips 261A-N.

Turning to FIG. 3, a block diagram of another embodiment of a memory module such as the memory module of FIG. 1 is shown. Components that correspond to those shown in FIG. 1 are numbered identically for clarity and simplicity. Memory module 150 of FIG. 3 includes a memory control hub 160 coupled to memory chips 261A through 261N via a memory bus 265.

Memory control hub 160 includes a control unit 240 coupled to a DRAM controller 250. DRAM controller 250 is coupled to memory chips 261A-261N and to a cache memory 175. The operation of the memory interconnect and aspects of the operation of the DRAM controller 250 of memory module 150 of FIG. 3 are similar to the operation of memory module 150 Of FIG. 2. However, in contrast to memory control hub 160 of FIG. 2, memory control hub 160 of FIG. 3 includes a cache memory 175 that includes a cache tag storage 175A and a cache data storage 175B. Differences in functionality are described further below.

In the illustrated embodiment, cache data storage 175B is configured to cache frequently accessed data stored within memory chips 261A-N. Likewise, cache tag storage 175A is configured to store address tags corresponding to the data stored within cache data storage 175B. Accordingly, data written to and read from memory chips 261A-N may be stored within cache data storage 175B. Thus, depending on whether the requested data is stored within cache memory 175, DRAM controller 250 is configured to initiate memory cycles to either memory chips 261A-261N or to cache data storage 175B in response to memory commands received by memory control hub 160. In one embodiment, cache memory 175A and 175B may be implemented using memory devices that are typically used for cache memory in a processor. For example, the memory devices may be in the static RAM (SRAM) or fast SRAM (FSRAM) family of devices. It is noted that although cache memory 175A-B is shown as part of memory control hub 160 (e.g., on the same device), it is contemplated that in other embodiments, cache memory 175A-B may be implemented as a separate device (e.g., on a different IC).

In one implementation, memory controller 105 is configured to initiate a memory access request to system memory 125. As described above, each memory module may be associated with a particular address space. Thus, when a memory request is received that has an address associated with memory module 150, using the tags stored within cache tag storage 175A, DRAM controller 250 is configured to determine whether the requested data is stored within cache data storage 175B. In the case of a read request, if the data is stored within cache data storage 175B (cache hit), DRAM controller 250 is configured to generate read cycles to cache data storage 175B. If, on the other hand, if the data is not stored within cache data storage 175B (cache miss), DRAM controller 250 is configured to generate read cycles to memory chips 261A-N.

Since the tags are stored within cache tag storage 175A, memory controller 105 may not have any a priori knowledge of whether a given read request will hit in the cache or not. Since read response latencies for cache hits and misses is typically different, the latency of a given read request is unknown to memory controller 105. To handle the unknown read response latency, memory controller 105 may include logic (not shown) that handles out-of-order read responses by tracking outstanding read responses.

It is noted that depending on the implementation, it may be necessary to ensure the correct ordering of other transactions. For example, it is important to ensure that write data (e.g., data that may be posted to cache data storage 175B) followed by a read of that data is not overwritten by a subsequent write to the same location prior to the read. In one embodiment, this functionality may be implemented in memory controller 105 while in other embodiments this functionality may be implemented in DRAM controller 250.

There are various ways to implement cache policies with respect to writing data. Thus, assuming that such transaction ordering protection is in place, irrespective of the location, if a write request is received that has an address associated with memory module 150, using the tags stored within cache tag storage 175A, DRAM controller 250 is configured to determine whether the requested data is stored within cache data storage 175B. In one embodiment, if the data is not stored within cache data storage 175B (cache miss), DRAM controller 250 may allocate a location within cache data storage 175B for the write data and possibly evict data already present. As described above, DRAM controller may include logic (not shown), such as write back buffers or eviction/victim buffers to support operation of cache data storage175A and tag storage 175B. DRAM controller 250 may write the received data into cache data storage 175B. The evicted data and the newly written data may be written back to memory chips 261A-N as determined by DRAM controller 250. If the write data is stored within cache data storage 175B (cache hit) and has not been flushed to memory chips 261A-N, DRAM controller 250 may simply overwrite the data within cache data storage 175B.

However, if the write ordering protection is not in place, and if the write data is stored within cache data storage 175B (cache hit) and has not been flushed to memory chips 261A-N, DRAM controller 250 may write the data back to memory chips 261A-N. DRAM controller 250 may then write the received data into the locations within cache data storage 250. Alternatively, DRAM controller 250 may move the dirty data already in cache data storage 175B into a write back buffer to be written back to memory chips 261A-N at some later point in time.

It is noted that to access the address space associated with memory chips 261A-N a memory access command code is used. An exemplary memory read packet is described below in conjunction with the description of FIG. 5.

Turning now to FIG. 4, a block diagram of one embodiment of a system including a serially connected chain of cache memory modules is shown. Components that correspond to those shown in FIG. 1 are numbered identically for clarity and simplicity. System 400 includes a host processor 410 coupled to a cache memory module 450A and a cache memory module 450B via a memory link 110A and a memory link 110B, respectively. Host processor 410 is also coupled to a system memory 425 via a memory interconnect 430. Cache memory module 450B is shown coupled to a memory link 110C, which may be coupled to an additional cache memory module (not shown) as desired to form a serially connected chain of cache memory modules that is coupled to processor host 410. It is noted that although two cache memory modules are shown in the chain, it is contemplated that any number of cache memory modules may be connected in this manner. It is further noted that components including a reference number followed by a reference letter may be referred to generally by the reference number alone. For example, when referring generally to all cache memory modules, reference may be made to cache memory module 450.

In the illustrated embodiment, memory links 110A-B and cache memory modules 450A-B provide any level of external cache capability to processor 410. For example, cache memory modules 45GA-B may be an external L3 or L4 cache and may provide a means of providing a very large external cache memory. Each cache memory module includes a cache memory control hub 460 which includes a cache memory 475. Processor 410 may include cache control logic (not shown) configured to generate cache transactions and to maintain cache coherency. Thus, the external cache memory provided by cache memory modules 450A-B may be used to cache frequently used data. That data may be stored within system memory 425 or any other storage. It is noted that system memory 425 may be representative of any type of system memory such as the system memory including the memory modules described above in conjunction with the descriptions of FIG. 1-3, for example.

It is further noted that additional memory modules (not shown in FIG. 4) may be connected downstream from memory module 450B. In such an implementation, the additional memory modules may be representative of the memory modules illustrated in FIG. 1 through FIG. 3 and may include memory chips. In one implementation, the additional memory modules may not include cache memories.

Cache memory transactions that are conveyed upon memory links 110A-C may be representative of the transactions described above in conjunction with the description of FIG. 1, above.

FIG. 5 and FIG. 6 illustrate exemplary memory access packets that may be conveyed on memory links 110A through 110C of FIG. 1. Turning now to FIG. 5, a diagram of one embodiment of a memory read packet is shown. In the illustrated embodiment, memory read packet 525 is 16 bits wide and includes six bit times or three bit-pairs. During bit time zero, the five-bit command code (e.g., 10h or 11h) is conveyed in bit positions 0-4. Bit positions 5-7 are reserved. An eight-bit tag is conveyed in bit positions 8-15.

During bit time one, the length of the data that should be returned conveyed in bit positions 0-5. In one embodiment, a value of 00h indicates no data, a value of 01h indicates two bit-pairs of data, a value of 02h indicates four bit-pairs of data, and so on. A zero length read results in an acknowledge packet (Ack) being returned to the requester. In one embodiment, a read of a half cache line or less may result in a short RdResp and a read of more than a half cache line may result in either a single long RdResp or two short RdResp. The cache line size may be programmed by software into the configuration registers of host 100 and each memory control hub160. Bit positions 6-7 are reserved. Address bits 39-32 of the requested location in DRAM are conveyed in bit positions 8-15.

During bit time two, the address bits 31-16 of the requested location in DRAM are conveyed in bit positions 0-15 and during bit time 3, the address bits 3-15 of the requested location in DRAM are conveyed in bit positions 3-15. Also during bit time 3, the packet priority is conveyed in bit positions 0-1. In one embodiment, the priority may be indicative of the priority of the packet relative to other requests. For example, one priority may be to delay all requests with lower priority even if they are already in progress and to execute this request ahead of them. Bit position 2 is reserved. During bit times four and five, bits 0-15 and 16-31, respectively, of a CRC are conveyed in bit positions 0-15.

Referring to FIG. 6, a diagram of one embodiment of a cache read packet is shown. In the illustrated embodiment, cache read packet 525 is 16 bits wide and includes 4 bit times or two bit-pairs. During bit time zero, the five-bit command code (e.g., 15h or 16h) is conveyed in bit positions 0-4. Bit positions 5-7 are reserved. An eight-bit tag is conveyed in bit positions 8-15.

During bit time one, the length of the data that should be returned is conveyed in bit positions 0-5. In one embodiment, a value of 00h indicates no data, a value of 01h indicates two bit-pairs of data, a value of 02h indicates four bit-pairs of data, and so on. A zero length read results in an acknowledge packet (Ack) being returned to the requestor. In one embodiment, a read of a half cache line or less may result in a short RdResp and a read of more than a half cache line may result in either a single long RdResp or two short RdResp. The cache line size may be programmed by software into the configuration registers of host 100 and each memory control hub160. Address bits 25-16 of the requested location in cache are conveyed in bit positions 6-15.

During bit time two, the address bits 15-0 of the requested location in cache 175 are conveyed in bit positions 7-16. During bit times three and four, bits 0-15 and 16-31, respectively, of a CRC are conveyed in bit positions 0-15.

It is noted that although only two types of packets were shown, other types of packets, which may correspond to the command codes listed in table 3 are contemplated. It is further noted that the illustrated packets are for discussion purposes only and that although the various fields of the exemplary packets are shown having a particular number of bits, it is contemplated that in other embodiments, the various fields of the each packet may include other numbers of bits as desired.

FIG. 7 is a block diagram of one embodiment of a computer system. Computer system 700 includes processor nodes 612A-612D each interconnected by coherent packet interface links 615A-D. Each link of coherent packet interface 615 may form a high-speed point-to-point link. Processor nodes 612A-D may each include one or more processors. Computer system 700 also includes an I/O node 620 which is coupled to processor node 612A via a non-coherent packet interface 650A. I/O node 620 may be connected to another I/O node (not shown) in a chain topology for example, by non-coherent packet interface 650B. Processor nodes 612A is illustrated as a host node and may include a host bridge for communicating with I/O node 620 via NC packet interface 650A. Processor nodes 612B-D may also include host bridges for communication with other I/O nodes (not shown). The non-coherent packet interface links formed by NC packet interface 650A-B may also be referred to as point-to-point links. I/O node 620 is connected to a pair of peripheral buses 625A-B.

FIG. 7 further illustrates respective system memories (e.g., 617A and 617B) coupled to processor nodes 612A and 612B. In the illustrated embodiment, processor node 612A and 612B are each illustrative of a host as shown in FIG. 1, and each system memory 617 may be implemented in the configuration described in conjunction with the descriptions of FIG. 2 and FIG. 3 above. Further, the interconnects between each of processor nodes 612A and 612B and their respective system memories 617 may be reflective of the memory interconnect including memory link 110C described above in FIG. 1 through FIG. 6. It is noted that in other embodiments, other numbers of processor nodes may be used. Further, it is contemplated that each of processor nodes 612C and 612D may be similarly connected to a respective system memory such as system memory 617, for example. As such, a cache memory of a memory control hub located within system memory 617A may cache data stored within either of system memories 617A or 617B, for example.

In the illustrated embodiment, each link of coherent packet interface 615 is implemented as sets of unidirectional lines (e.g. lines 615A are used to transmit packets from processing node 612A to processing node 612B and lines 615B are used to transmit packets from processing node 612B to processing node 612C). Other sets of lines 615C-D are used to transmit packets between other processing nodes as illustrated in FIG. 7. The coherent packet interface 615 may be operated in a cache coherent fashion for communication between processing nodes (“the coherent link”). Further, non-coherent packet interface 650 may be operated in a non-coherent fashion for communication between I/O nodes and between I/O nodes and a host bridge such as the host bridge of processor node 612A (“the non-coherent link”). The interconnection of two or more nodes via coherent links may be referred to as a “coherent fabric”. Similarly, the interconnection of two or more nodes via non-coherent links may be referred to as a “non-coherent fabric”. It is noted that a packet to be transmitted from one processing node to another may pass through one or more intermediate nodes. For example, a packet transmitted by processing node 612A to processing node 612C may pass through either processing node 612B or processing node 612D as shown in FIG. 7. Any suitable routing algorithm may be used. Other embodiments of computer system 700 may include more or fewer processing nodes than the embodiment shown in FIG. 6.

One example of a packet interface such as non-coherent packet interface 650 may be compatible with HyperTransport™ technology. Peripheral buses 625A and 625B are illustrative of a common peripheral bus such as a peripheral component interconnect (PCI) bus. It is understood, however, that other types of buses may be used.

It is further noted that other computer system configurations are possible and contemplated. For example, it is contemplated that the system memory configuration described above may be used in conjunction with a computer system employing a processor chipset that includes a Northbridge. In such an embodiment, a memory controller within the Northbridge may serve as the host.

Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. 

1. A system comprising: a host; and one or more memory modules coupled serially in a chain to said host; wherein at least one of said memory modules includes a cache for storing data stored in a system memory.
 2. The system as recited in claim 1, wherein said system memory includes a respective plurality of memory chips mounted on given memory modules of said one or more memory modules.
 3. The system as recited in claim 2, wherein each memory module includes a memory control hub including a controller configured to determine whether data associated with a received memory request is stored within said cache.
 4. The system as recited in claim 3, wherein a given cache includes a storage for storing cache tags corresponding to said data stored within said given cache.
 5. The system as recited in claim 4, wherein said controller is configured to access said cache in response to determining that said data associated with a received memory request is stored within said cache.
 6. The system as recited in claim 2, wherein said host includes a memory controller configured to generate memory requests to said one or more memory modules.
 7. The system as recited in claim 6, wherein said memory controller includes a storage for storing cache tags corresponding to said data stored within each cache.
 8. The system as recited in claim 7, wherein said memory control hub includes a controller configured to access said plurality of memory chips in response to receiving a memory command having a memory address that matches a memory address associated with said plurality of memory chips.
 9. The system as recited in claim 8, wherein said controller is further configured to access said cache in response to receiving a memory command having a memory address that matches a memory address associated with said cache.
 10. The system as recited in claim 6, wherein said memory controller is further configured to issue a memory read request transaction prior to receiving a response to a previous memory read request transaction.
 11. The system as recited in claim 1, wherein said one or more memory modules is coupled serially in a chain to said host via a plurality of memory links, wherein each memory link includes an uplink for conveying transactions toward said host and a downlink for conveying transactions originating at said host to a next memory module in said chain.
 12. The system as recited in claim 11, wherein said uplink and said downlink are each a unidirectional link including a plurality of signals configured to convey transactions using packets that include control and configuration packets and memory access packets, wherein at least a portion of packets include control, address and data information, and wherein said control, address and data information share the same wires of a given link.
 13. The system as recited in claim 12, wherein each of said plurality of signals of each of said unidirectional links uses differential signaling.
 14. The system as recited in claim 13, wherein said packets are conveyed on said uplink and said downlink using 16 bit positions to convey said control, address and data information.
 15. A memory module comprising: a cache configured to store data stored in a system memory; a memory control hub coupled to control access to said storage; wherein said memory control hub is configured to receive memory transactions originating at a processor via a first downlink and to transmit said memory transactions via a second downlink to another memory module independent of decoding said transactions.
 16. The memory module as recited in claim 15, wherein said system memory includes a respective plurality of memory chips mounted said memory module.
 17. The memory module as recited in claim 16, wherein said memory control hub includes a controller configured to determine whether data associated with a received memory request is stored within said cache.
 18. The memory module as recited in claim 17, wherein said cache is further configured to store cache tags corresponding to said data stored within said cache.
 19. The memory module as recited in claim 18, wherein said controller is configured to access said cache in response to determining that said data associated with a received memory request is stored within said cache.
 20. The memory module as recited in claim 16, wherein said processor includes a memory controller configured to generate memory requests to said memory module.
 21. The memory module as recited in claim 20, wherein said memory controller includes a storage for storing cache tags corresponding to data stored within said cache.
 22. The memory module as recited in claim 21, wherein said memory control hub includes a controller configured to access said plurality of memory chips in response to receiving a memory command having a memory address that matches a memory address associated with said plurality of memory chips.
 23. The memory module as recited in claim 22, wherein said controller is further configured to access said cache in response to receiving a memory command having a memory address that matches a memory address associated with said cache. 